Term Paper: Caspian Sea

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Ecology/Caspian Sea

Community Concepts

The concept of community is one that has defied definition for centuries, even among individuals in the field of ecology. While all agree that a community involves a group of species together in the same area, and that those species must co-exist and interact competitively for resources, the actual definition of a community concept is varied (Allen, 1998). However, there are two viewpoints that appear most often in today's concept of community, and this section will discuss those viewpoints individually.

The first concept of community is that of the "discrete unit" by Frederic Clements. According to this concept, a community is like a superorganism, where the species of the area are interconnected like parts of a body (Ricklefs, 2001). Each species in the community has co-evolved, or evolved along with one another in such ways as to enhance the independent functioning of one another. As this holistic view describes, these independent species are understood in the sense of community only by the contributions each species makes to the dynamics of the entire system (Allen, 1998). It is precisely these contributions, made by independent species in an interactive way through co-evolution, that allow communities to develop.

Clements' "discrete unit" concept of community describes a closed community. In other words, each discrete unit or community is distinct to its self, and has very specific boundaries, or ecotones. Distributions of species within the community are coincident, rather than independent (Allen, 1998). It is only through succession, or the orderly sequence of changes in communities, that a stable community is possible (Allen, 1998).

The second concept of community is derived from Henry Gleason's idea of the loose assembly of species (Ricklefs, 2001). This concept revolves around the idea that a community is an association of species whose requirements and adaptations to the area allow them to live with one another under specific conditions. Under this idea, the species that cohabitate do so by coincidence, rather than co-adaptation (Allen, 1998). Gleason's concept is one of an open community, as well, where no community has boundaries.

Under Gleason's community concept, a community's structure and functions are only expressions of the interactions between the species, not due to an organization of those species. In this open community, then, distributions of species are independent of one another, and without definition (Allen, 1998). They do not exist out of necessity, as in the "distinctive unit" concept, but rather out of coincidence.

Regardless of which concept is assumed, there are some agreed upon ideas about biological communities. For example, in any community, closed or open, organisms coexist with one another, and are linked to one another by their interactions, giving any concept of community both a special and a functional definition. Further, any ecological effect or evolutionary effect on the community affects all species, since all species interact with one another.

In the Caspian Sea, there are many spatial areas that could be considered ecological communities. In particular, the area of the southern Caspian is home to a particularly rare community of jellyfish, and this community will be examined throughout the rest of this analysis. Within this community, the Beroe ovata jellyfish has been introduced to combat the comb jellyfish. The comb, in turn, relies on the zooplankton in the area, which relies on the phytoplankton for food. All creatures in this community rely on the oxygen produced in the water by these members of the community (Jeffress and Steimle, 1990).

Food Webs

Generally speaking, the idea of a food web relates to the concept of who eats whom within any given ecosystem, or to the flow of energy in an ecosystem (Allen, 1998). A true food web outlines the feeding relationships between all species in a given ecosystem, and thus can be far more representative than a standard food pyramid, but also much more complex. While this certainly leads to the potential of showing diversity and differentiation within a community structure, the lack of a definition for "community structure" means that for different community concept definitions, different food web results may emerge (Oceanlink, 2005).

This being noted, there are some basic principles behind food web analysis. First, in any community, the more complexity that can be introduced into the food web, the more stable the community will become. When provided with alternative resources, any population will depend less on one single resource, thus making it possible to survive if that single resource is destroyed. Additionally, if the energy flowthrough of an ecosystem has multiple pathways, a single disruption simply reroutes the energy through alternate channels (Ricklefs, 2001).

Furthermore, characterization of communities can occur based on the number of species and feeding links per species. As community diversity increases, the number of trophic levels, such as the herbivore or omnivore levels, and the number of guilds, such as the leaf eaters of the herbivore trophic level, also increase. These trophic levels, then, help influence the food web by describing either a top down or bottom up energy chain.

In top down chains, predators depress populations of other animals, such as herbivores, resulting in a "green" earth. In bottom up chains, plants resist consumption by toxins and digestion inhibition, resulting also in a "green" earth. In both cases, predation and production form intricate parts of the food web (Ricklefs, 2001).

A prime example of a problematic food web can be seen in an analysis of the southern Caspian Sea, in particular relationship to the Mnemiopsis leidyi, or comb jellyfish. A rampant population growth of this jellyfish in the southern Caspian in 2000 had drastic effects on the food web of the ecological community. These jellyfish eat mollusk larvae, and other small zooplankton. These zooplankton, in turn, feed on microscopic algae blooms and other phytoplankton, creating an effective food web that maintains the level of oxygen in the water, thus creating a co-existent community. If levels of all species remain stable, the community will be stable (Jeffress and Steimle, 1990).

However, overabundance of the Mnemiopsis leidyi creates a large problem for this community, since the food web is not complex. If the jellyfish feeding drastically reduces the numbers of zooplankton, then the zooplankton do not effectively consume the phytoplankton. This phytoplankton, then, sinks to the bottom of the Caspian, and dies. As a result, the oxygen levels of the waters decrease, further lowering the zooplankton levels. As zooplankton levels decrease, the food supply of the primary predator, the Mnemiopsis leidyi, is also reduced, thereby starving the jellyfish (Jeffress and Steimle, 1990).

Clearly, this food web is not diversified, and thus, not stable. If any energy pathway in this web is destroyed, the entire web detangles. If, instead, the web had some diversification, any link could be altered, and other links would simply increase in production. This is the basic premise of the food web.

Succession

Clements' theory of succession refers to the sequence of changes, or sere, in a community initiated by a change or disturbance. For any community, a continuous flux of energy and material cycling is expected. Following a drastic change or disturbance of these dynamics, the community rebuilds its self, returning to its normal composition and structure. Once a species achieves its highest level of organization, the community can be called a climax community (Goldsmith, 1985).

Generally speaking, there are two forms of succession. Primary succession occurs when plants develop in areas previously without vegetation, such as in sand dune areas and lava flows. Secondary succession, on the other hand, occurs when a disturbance in the normal dynamics of a community initiates a regeneration of that community. These secondary successions can be created by weather introduction of new species, fire, a shift in predatory populations, or other phenomenon (Ricklefs, 2001).

According to Joseph Connell and Ralph Slatyer, there are three primary mechanisms involved in succession, those of facilitation, inhibition, and tolerance. With facilitation, each species allows the next species to grow to their own climax (Goldsmith, 1985). For example, in the Caspian Sea, the appearance of phytoplankton in the water gives off oxygen, and provides a food source for zooplankton. Without the phytoplankton, the zooplankton would be unable to develop. The climax, then, is reached when the balance between the phytoplankton and zooplankton are such that alterations no longer occur (Jeffress and Steimle, 1990).

Inhibition, on the other hand, describes the succession that occurs when one species hinders the growth of another species. It is only through the death of a species and the replacement of that species that succession can occur, creating a longer living species (Goldsmith, 1985). In the case of the Caspian Sea, this can be seen with the introduction of the Beroe ovata jellyfish. This jellyfish feeds exclusively on the comb jellyfish that, as noted, has caused a rapid alteration of the ecosystem by depleting the zooplankton and consequently, the phytoplankton in the southern Caspian. With the introduction of the Beroe ovata, which inhibits the growth of the comb, the other species in the area are given the opportunity to reestablish… [END OF PREVIEW]

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